Traditionally, the touch sensor of a capacitive touchpad is realized by a printed circuit board (PCB). However, the opaqueness of the PCB restricts applications of the capacitive touchpad in cell phones, personal digital assistants (PDAs), multi-media display panels and other electronic products. Transparent electrically conductive films, such as indium tin oxide (ITO) and indium zinc oxide (IZO), can be used to replace the PCB traces for transparent applications. Alternatively, a transparent membrane printed with conductive carbon paste or silver ink trances may implement applications of a capacitive touchpad for electronic products where the touchpad is intended to reveal through a bottom side thereof. However, unlike the PCB trace whose impedance is so low as to be negligible, the trace made of ITO, IZO, conductive carbon paste, silver ink or the like exhibit a significant impedance, which will result in errors in sensed values of the touch sensor and disadvantageously affect touch positioning by the capacitive touchpad. As shown in FIG. 1, a capacitive touchpad 100 includes a touch sensor 110 and a controller 120. The touch sensor 110 shown therein is a two-dimensional one, which has sensing lines arranged in two directions, namely the group of X1, X2, . . . , Xm, . . . , XM and the group of Y1, Y2, . . . , Yn, . . . , YN. Typically, the two directions of the sensing lines are referred to as X direction and Y direction for convenience's sake. If the touch sensor 110 is realized by a PCB, the sensing lines X1, X2, . . . , Xm, . . . , XM and Y1, Y2, . . . , Yn, . . . , YN are copper traces on the PCB. If, for transparent applications, ITO or IZO is used to make the sensing lines X1, X2, . . . , Xm, . . . , XM and Y1, Y2, . . . , Yn, . . . , YN, then the substrate for the sensing lines can be made of glass, plastic or other transparent materials. If the sensing lines X1, X2, . . . , Xm, . . . , XM and Y1, Y2, . . . , Yn, . . . , YN are conductive carbon paste or silver ink, the substrate is a transparent membrane. The controller 120 is a semiconductor chip installed on a flexible printed circuit board (FPC) 115, and is connected to the sensing lines X1, X2, . . . , Xm, . . . , XM and Y1, Y2, . . . , Yn, . . . , YN by metal wires 125 printed on the FPC 115. The controller 120 has a detector circuit therein, to detect the capacitance variations along the sensing lines X1, X2, . . . , Xm, . . . , XM and Y1, Y2, . . . , Yn, . . . , YN. The detected capacitance variation is referred to as a sensed value, from which a position of an object touched on the touch sensor 110 can be determined.
In further detail, as shown in FIG. 2, a sensing line has many capacitive sensor pads 130 to 148 thereon. If this sensing line has an impedance so low as to be negligible, the sensed values generated by an object touch on anywhere of the sensor pads 130 to 148 are substantially equal, and allow the controller 120 to precisely determine, according to preset reference values, whether or not an object touch has been made. On the contrary, if the electric resistance of this sensing line is too large to be ignored, the sensing line of FIG. 2 will have an equivalent circuit as shown in FIG. 3, which will produce a resistor-capacitor (RC) filtering effect on the sensed values generated therefrom. As a result, referring to FIG. 4, when an object touches the sensing line at different sensor pads 130 to 148, the sensed values detected by the controller 120 will be different from each other and are attenuated with the distance between the controller 120 and the touched sensor pads 130 to 148. Consequently, there is a great difference between the sensed value corresponding to the nearest sensor pad 130, which is adjacent to the controller 120, and the sensed value corresponding to the farthest sensor pad 148, which is away from the controller 120. The attenuation of the sensed values due to actual impedance makes it difficult to make adjustments to the capacitive touchpad, or even impossible to detect a capacitance variation if a thicker medium is used in the touchpad. Moreover, even if an object touches a same sensing line, the sensed values corresponding to different sensor pads may be so significantly varied as to increase the chances of error actions resulted from misjudgments by the controller.
The problem resulted from the attenuation of the sensed values due to the impedance of a sensing line itself can be minimized by arranging all the sensing lines of a capacitive touchpad in an interleaving manner so as to homogenize the resistance/capacitance distribution of the sensing lines. However, for interleaving sensing lines arrangement, the sensing lines in X and Y directions are drawn to the controller from two opposite ends thereof and result in a rather complicated wiring layout. Moreover, as shown in FIG. 5, if a touch sensor 210 has a rectangular shape, and the sensing lines in X and Y directions are drawn to a controller 220 from two opposite ends thereof, the excessively long sensing lines 230 to 234 not only increase the difficulty in wiring layout, but also produce additional parasitic resistances, which will further increase the difficulty in signal processing as well.